The present invention relates to a method of producing a thin film magnetic head. More particularly, it relates to a method of producing a thin film magnetic head that involves a machining step of a gap depth of a thin film magnetic head, providing eventually a magnetic head having stable electrical characteristics irrespective of variance inherent to individual heads that occurs during a production process.
Recently, electronic computers have come to possess information processing capacity comparable to that of large-scale computers of a generation ago. On the other hand, down-sizing has made a remarkable progress. With such a background, function of magnetic disk devices has drastically been improved so as to satisfy the demands for a smaller size and a greater capacity. For these reasons, so-called "thin film magnetic heads", which are produced by forming a ultra-fine magnetic head structure on a substrate by the application of photolithography technique, have gained a wider application to further improve a recording density.
To produce this thin film magnetic head, a large number of thin film magnetic conversion devices are formed in lot on a substrate and then head slider machining is carried out in a block unit having these thin film magnetic conversion device mounted thereto. In this way, the production cost per head can be reduced and a higher recording density can be accomplished.
In the thin film magnetic head of this kind, high density recording/reproduction performance is affected greatly by a gap depth which is the distance between a position of a distal end of an insulating layer (GO in later-appearing FIG. 2B) between upper and lower magnetic layers and a floating surface opposing a recording medium. Recently, dimensional accuracy almost approximate to the limit of machining technology for forming the gap depth has been required for the gap depth with an improvement in performance required for the thin film magnetic head. In other words, since a slight dimensional error greatly affects performance of the thin film magnetic head, the gap depth is the greatest factor for deteriorating the production yield of the thin film magnetic head, and solution of this problem is the most critical problem during the production of the thin film magnetic head.
FIG. 10 of the accompanying drawings illustrates appearance of a thin film magnetic head slider, and FIGS. 2A to 2C show a detailed structure of this thin film magnetic conversion device. As depicted in FIG. 10, a thin film magnetic conversion device 9 is generally mounted to a medium flow-out end of a head slider 1 made of a ceramic material. In this example, two thin film magnetic conversion devices are shown mounted to one slider. Reference numeral 9a denotes a lead wire and 9b denotes a terminal. As shown in detailed structural sectional views of FIGS. 2A to 2C, the thin film magnetic conversion device 9 includes a magnetic yoke comprising lower and upper magnetic layers 5 and 6 formed on a base layer 2, and a conductor coil 8, an insulating layer 7 and a magnetic gap film 3 that are formed inside the magnetic yoke. This structure is covered as a whole with a protective film 4 and is formed on a substrate (head slider) 1. The dimension of the gap depth is a distance G from a position of a distal end GO of the insulating layer 7 (a position at which the gap between the lower magnetic layer 5 and the upper magnetic layer 6 starts expanding) to a surface 1F which is an air lubrication floating surface to a magnetic recording medium. This dimension of the gap depth is a dimension inside the thin film magnetic conversion device, and cannot be inspected directly from outside. Incidentally, FIG. 2B is a sectional view of an induction type thin film magnetic head, and FIG. 2C is a sectional view of a composite type thin film magnetic head.
Machining of this gap depth is carried out in the following way. First of all, the substrate 1 on which a large number of thin film magnetic conversion devices are formed as shown in FIG. 3 is cut into blocks 1B in a unit of a plurality (generally, a dozen to 20 or 30) of thin film magnetic conversion devices, and then the air lubrication floating surface 1F with respect to the magnetic recording medium shown in FIG. 10 is finished into a precise and uniform dimension by polishing, etc., between the blocks and inside each block. (By the way, each pair of thin film magnetic conversion devices 9 shown in FIG. 3 are cut and one slider shown in FIG. 10 is constituted.) However, the dimension of this gap depth cannot be inspected directly during machining as described above and its absolute value is ultra-small. Accordingly, it has been extremely difficult to precisely and uniformly control it between the blocks and inside each block.
To solve the problems described above, JP-A-63-29315 proposes a method which disposes a plurality of signal detection circuits for detecting the point at which a gap depth reaches a predetermined gap depth dimension, and precisely controls the absolute value of the gap depth dimension. Further, the Applicant of this invention proposes a method in JP-A-3-27008 (Japanese Patent Application No. 2-68268) which always monitors a gap depth dimension of a plurality of thin film magnetic heads inside a given block by the application of the signal detection circuits described above and stops machining at the point at which the distribution of the dimension inside the block is in the best conformity with dimensional tolerance.
However, variance of the substrates 1 (FIG. 3) as a production unit in pre-stages for forming a magnetic head structure on the substrate and various production variances occurring inside the plane of the substrate are also closely associated with the gap depth and affect performance of the thin film magnetic head.
As between the recording and reproduction performance of the thin film magnetic head, it is the recording characteristics of magnetic signals to a magnetic recording medium that is affected particularly greatly by the gap depth. Besides the gap depth, the shape constituting the magnetic head structure such as a thickness of a magnetic film of a magnetic core, and various other factors also affect the recording characteristics. To eventually obtain predetermined performance required for the magnetic head, it is necessary to distribute tolerance in accordance with all of these variance factors, to allot tolerance to each factor item and to fabricate the head within the tolerance of each factor.
Since the dimension of the gap depth has been determined in consideration of all the variance factors as described above, considerably fine standardized rating values of tolerance have been set to the gap depth. When the gap depth is examined in the unit of the individual thin film magnetic head, however, the required optimum center value of the dimension of the gap depth does not coincide in many cases with the standardized rating value depending on the machining conditions of the pre-fabrication stage. For this reason, there is a certain predetermined level limit to the yield in the aspect of signal recording performance of the head.